This research sheds newlight on the much-debated link between agricultural productivity and development.We
do so by estimating the causal impact of a large shock to agricultural productivity—the introduction of the heavy
plowin theMiddle Ages—on long run development. Webuild on thework of LynnWhite, Jr. (1962), who argued
that it was impossible to take proper advantage of the fertile clay soils of Northern Europe prior to the invention
and widespread adoption of the heavy plow. We implement the test in a difference-in-difference set-up by
exploiting regional variation in the presence of fertile clay soils. Using a high quality dataset for Denmark, we
find that historical counties with relatively more fertile clay soil experienced higher urbanization after the
heavy plow had its breakthrough, which was around AD 1000. We obtain a similar result, when we extend the
test to European regions. Our findings substantiate that agricultural productivity can be an important driver of
long-run development.